I agree with some of the replies on your other thread:
- give positive reinforcement (acknowledgement/praise/massive praise) when he expresses his own opinion about anything.
- start off with limited choices, then widen out.
Open-ended questions might freak him out - he just might not know how to answer them.
In my ds's case (HFA) when he was 3yo he didn't know how to answer the question "what's your name". Now he's 5yo and has progressed an enormous amount, he still struggles with questions which are open-ended/abstract. I can imagine him doing the same as your ds, and copying his best friend. (I'd consider this a good strategy for my ds but of course we're in a very different situation from you.)
I'd definitely ask the teacher to start with questions that are more either/or.
Also is there any chance the teacher could give you the list of topics they will be covering at carpet time? If you know what the subjects are you could start chatting about them at home - not to 'prep' him on what to say, but to get him interested in the subjects and hearing all the different things to say about them (you and ds1 could model this for him?). If he knows a bit more about the subjects being discussed he'll probably have more understanding, more motivation, and more confidence.